This invention relates to hydraulic systems, and in particular to a sensor for incorporation into a hydraulic motor for measuring rotational displacement of the motor shaft.
The agriculture industry has traditionally been one of the leading users of hydraulic systems. The abilities of hydraulic components to produce high amounts of power in a small space and to reach remote applications are two of the major advantages of hydraulic systems for the use in industrial and agricultural applications. Hydraulic actuators can be placed in locations remote to the power source and thus are commonly used on farm implements and in industrial plants. Hydraulic components can be controlled within a certain degree of precision by an operator or by hydraulic feedback through the system.
With the development of the microprocessor and the microcontroller, a new dimension of feedback control has resulted. The high speed microcontroller can process thousands of pieces of data per second with extreme precision, limited only by the components used in the electronic circuit. The major drawback in the use of electronic control has been the limitations imposed by the interfacing units used to provide the microcontroller with needed information about the process. The traditional interfacing units in hydraulic systems have been the pressure transducer, flow meter, tachometer, numerous linear position sensing devices, and various other transducers. Although many of these transducers provide high accuracy, most suffer from high cost. For example, the pressure transducer requires its own electronic circuitry which increases the cost and is also packaged for use in a variety of applications which in turn limits its use in any single situation.
The number of types of transducers used in the past has been limited, which has created interest in the area of sensor development. Sensors are being developed for a specific task, such as sensing implement draft using strain gages on a link of an agricultural tractor's three point hitch. There is a need to develop sensors which can be used for a specific purpose and provide information on multiple variables using a single signal. One such need is for a sensor which can produce information about the rotational displacement of a hydraulic motor. In order for this type of sensor to be useful in agricultural and industrial situations, it must have low cost, be sufficiently accurate, and be able to withstand the environments in which hydraulic motors are used.
Control of mass flow rate on a conveyor is one example of the potential use of this type of rotational displacement sensor. If the flow of material exiting a conveyer is to be periodically interrupted, then the amount coming off the conveyer is dependent on how far the driving motor has turned. In this situation, a low cost sensor with sufficient accuracy is very desirable.
An example of a future application might be the rotational control of agricultural robots via motor sensors. Many types of agricultural robots are going to be developed in the future. Some of these, such as robots used for spraying, will require a low cost sensor which has sufficient accuracy and which is inherently shielded from the weather, temperature, and abuse associated with agricultural uses.
Past uses of transducers have been to instantaneously measure a specific variable, such as torque. Many additional applications are possible for transducers which can produce a signal or signals corresponding to a variety of variables. A transducer to sense two of the other variables needed to be measured in a hydraulic motor, speed and torque is disclosed in the U.S. patent application Ser. No. 562,436 of Gary Krutz and Mark Evans, now U.S. Pat. No. 4,593,555. The transducer comprised a full bridge strain gage network which was used to sense pressure inside the motor. The speed was determined by the frequency of the square wave output divided by the number of pressure waves per revolution. New applications could be developed if a transducer was capable of measuring rotational displacement as well as speed and torque.